Several types of light modulating liquid crystalline materials are currently known. Ideally, these materials provide desirable features such as high contrast, fast switching times between different optical states and wide viewing angles. Some other desirable features are low power consumption to switch and maintain optical states, mechanical, thermal, and electrical stability, and ease of fabrication.
One category of organic material that exhibits most of the above desirable characteristics are ferroelectric liquid crystals which are also referred to as chiral smectic C liquid crystals. Smectic liquid crystals consist of long, rod-like molecules arranged in layers. In instances where the molecules are tilted at an angle to the layers, they are called smectic C liquid crystals. When a chiral group is incorporated into the material's molecules or when a chiral substance is added to the smectic liquid crystal, the molecules form a helical pattern along a direction perpendicular to the layers. These structures are known as ferroelectric liquid crystals. It is known that ferroelectric liquid crystals provide extremely fast switching times compared to nematic or cholesteric liquid crystal materials. Additionally, the viewing angles of ferroelectric displays are much wider than most other types of liquid crystal displays.
In particular, the chiral smectic C* mesophase, which is a ferroelectric liquid crystal (FLC) phase, is mainly used for light modulation in surface stabilized FLC (SSFLC) devices, ferroelectric gels, deformed helix formation (DHF) effect devices and polymer dispersed FLC (PDFLC). SSFLC and DHF devices employ pure FLC filled in a cell comprising two flat substrates each with alignment layer. Ferroelectric gels include pure FLC, which has a relatively small amount of polymer dissolved therein, oriented by the substrates with alignment layers, wherein polymer is evenly distributed throughout the FLC and becomes part of the optically active aspects of the cell. PDFLC devices, which utilize droplets of FLC embedded in a polymer matrix induced by phase separation, can also be used as light modulating elements. Alignment of a FLC director inside the droplets is obtained by highly anisotropic action of the polymer matrix or by external forces, such as an applied electromagnetic field or shearing.
As those skilled in the art will appreciate, devices using SSFLC and DHF materials and ferroelectric gels require that a relatively small cell gap, about 1.5 .mu.m, be utilized to obtain optimum contrast and transmission. Moreover, high uniformity of cell spacing between the substrates is essential. In other words, the device will not perform adequately unless the material is uniformly oriented between properly and precisely separated cell substrates. Still another disadvantage of these devices is that they are highly susceptible to mechanical, thermal, and electrical shock or stress. Yet another disadvantage is that these materials tend to develop textural defects, such as zig-zag defects, during device fabrication. It is also difficult to obtain desirable properties such as grey scale in SSFLCs.
The PDFLC light modulating devices possess grey scale properties and are free of several of the disadvantages of the other three devices; however, they have their own particular disadvantages. In particular, the refractive indices of both the polymer and FLC, must match to avoid light scattering from the droplets' surface. Another disadvantage is that the PDFLC devices often require high applied voltages for optimum performance and as such, are not conducive to control by low voltage electronic drivers.
In regard to optical appearance, pure SSFLC cells using a polarizer and a crossed analyzer illuminated with white light have a contrast ratio of greater than 100 and a transmission value of about 80 percent of the maximum possible. But as noted previously, SSFLC devices are very sensitive to mechanical shock which can easily damage liquid crystal alignment and render them unusable.
Several other liquid crystal materials likely to be used for light modulation, are antiferroelectric liquid crystals (AFLC), cholesteric and nematic liquid crystals. AFLCs are also sensitive to mechanical deformations and thermal shocks and require a small but uniform cell gap. Being relatively unexplored for applications, their physical parameters remain to be optimized for electrooptical applications. Main disadvantages of nematic and cholesteric devices are their slow response and narrow viewing angle.